It is known that a key element for achieving laboratory-grade precision and accuracy is the use of an aqueous-based calibration solution containing known concentrations of all analytes to be tested. Typically, a single point calibration is performed immediately prior to the measurement of a clinical sample. This ensures reproducible results from instrument-to-instrument and from sensor-to-sensor. The current sensor or “optode” cassettes used in certain optical sensor based instruments, such as OPTI® instruments of Osmetech® (Roswell, Ga.), are wet-stored in the calibration solution, thus no waiting period is needed for sensors to wet-up from the dry state prior to use. The principal disadvantage for wet-storage is the resulting sensor “shelf-life” limitation, especially for certain sensors such as biosensors with hydrolytically unstable components. In addition, a requirement of being able to store wet calibration solution significantly complicates cassette design, thus increasing costs.